docs: def hangup(self):
          """ End the phone call.
          
          Does nothing if the call is already inactive.
          """
          if self.active:
              self._gsmModem.write('ATH')
              self.answered = False
              self.active = False
          if self.id in self._gsmModem.activeCalls:
              del self._gsmModem.activeCalls[self.id]